Displays, such as LCD, plasma, OLED, CRT, or based on other types of technologies, are all subject to manufacturing tolerances and intentional variations in their color attributes. For example, many of today's color monitors are designed to render “white” at a color temperature of about 6500° K, whereas LCD TV displays can be designed to render “white” at a color temperature of about 10,000° K. The appearance of white on an LCD TV can therefore appear “bluer” than white on a monitor, even if they are presented and driven with the same electronic input signal.
Creating accurate color images on a display is especially important in broadcast, graphics, and medical applications. These applications depend on accurate and consistent color image rendition to assess a scene composition or the results of hard-copy printing, or to determine the health of a patient. For example, subtle differences in the colors in an image may establish the level of appeal of a scene or the level of oxygen in ones blood.
Most display images are created by mixing together various combinations of red, green, and blue light. These three colors are considered to be the display's primary colors. If the precise color of these primaries can vary from display to display, the result of mixing two or more of these primaries together will be variable as well. Therefore, for repeatable image creation, consistency and repeatability in generating the primaries need to be established.
The techniques used for manufacture of displays with repeatable primaries have improved significantly over time. Internationally recognized color standards have been developed that define color primaries. Nevertheless, because of the display manufacturers' need to produce displays economically and efficiently, there remain color differences between manufacturing lots, between display models from the same manufacturer, and between manufacturers themselves.
FIG. 1 illustrates a color capability, or color gamut chart, which provides a graphical representation of the colors a display can produce and is graphically represented on an x,y chart. Possible colors that a specific display can produce are contained within triangular area 100 as shown in FIG. 1. The vertices 102, 104, and 106 of triangle 100 represent points within the red, green, and blue color regions of the color gamut chart, for example. The horseshoe shaped region 108 in FIG. 1 illustrates the entire color spectrum that can be seen by a human being. Any colors outside of triangle area 100 within horseshoe shaped region 108 may not be produced by the display. Each display color is created upon receipt of an input electronic signal which contains information (data) about the relative amounts of the three primaries; in this case, red, green, and blue. If a second display has primaries whose colors differ from a first display, the second display's resultant color (and image, graphic, or video appearance) will differ as well. Minor primary differences will yield minor resultant color differences, but since the human eye is extremely sensitive to color variations, the differences will frequently be noticeable, especially to those with professional experience and training.
Previous methods of modifying an electronic input to a display so that its resultant colors match those of a standard display include the following. In FIG. 2, rin, gin, and bin represent input signals of a display. Signal gain and/or offset circuits for red, green, or blue can be adjusted (e.g., manually adjusted) to produce rout, gout, bout such that when input to the display, its color will match that of a standard display. Although a specific resultant color within the gamut can be matched, the entire gamut may not be matched with this technique.
FIG. 3 illustrates another prior method for modifying an input, including modifying red, green, and blue signals with a 3×3 matrix multiplication. This method is broadly used to translate signals from one color space to another (e.g., from RGB to YUV and vice-versa). However, since “a” through “i” are constants, the transform is linear but does not produce a revised gamut that simultaneously retains a consistent display luminance. Values for “a” through “i” that vary as a function of rin, gin, bin are desired, but may be difficult to determine.
Another prior method for modifying an input includes using large Look Up Tables (LUTs) where every rin, gin, bin combination is represented by a corresponding corrected rout, gout, bout. While the output data may be exact, the memory required, as well as the access speed, may become prohibitive. This is especially so as input color depth grows from 24-bit to 30-bit and beyond. For example, the memory required for a 24-bit LUT must be greater than 400 Mb and have an access time less than 8 ns (to drive a 1080 p display). For a 30-bit color depth, the memory size grows to more than 32 Gb.
Therefore, novel methods are needed to modify an electronic input to a display so that the resultant colors will match those of a “standard” display.